The magnetic resonance imaging (hereinafter, abbreviated as “MRI”) device irradiates (applies) a radio frequency magnetic field with a specific frequency to a subject placed in a static magnetic field, thereby exciting nuclear magnetization of an atomic nucleus in hydrogen or the like, contained in the subject, detecting a nuclear magnetic resonance signal generated from the subject, and acquires physical and chemical information. In addition to magnetic resonance imaging for imaging the nuclear magnetic resonance signal, a measuring technique using the MRI device includes MRS measurement, which makes use of a difference in resonance frequencies due to a chemical coupling between various molecules containing hydrogen nucleus (hereinafter, referred to as “chemical shift”), so as to separate nuclear magnetic resonance signals obtained from one to several regions into signals by molecule, thereby acquiring information of metabolites (see patent document 1, for instance).
The measuring method described in the patent document 1 is referred to as “PRESS method”, and it is the most frequently used method as the MRS measurement at present, which localizes a region targeted for spectral measurement. In this PRESS method, after applying a gradient magnetic field (GC) pulse for selecting a predetermined slice, together with a radio frequency magnetic field (RF) pulse for exciting nuclear magnetization, GC pulses for selecting slices in two directions being orthogonal to the predetermined slice are applied respectively, together with an RF pulse for inverting the nuclear magnetization, and thereafter, measuring a nuclear magnetic resonance signal from a region (voxel) where the three slices intersect with one another. Then, the nuclear magnetic resonance signals thus measured are subjected to the Fourier transform in the time axis direction, and magnetic resonance spectra signals are obtained.
The MRS measurement has an outstanding advantage that it is capable of measuring a metabolite inside a human body noninvasively, this advantage being incomparable with other measuring methods, and it is spreading gradually in the clinical field in recent years. Particularly, the head region (brain) is rated as the most suitable organ for the MRS measurement. This is because the head region has an almost spherical shape, facilitating enhancement of static magnetic field homogeneity, and it is relatively easy to suppress enormous signals of water which hinder acquisition of extremely feeble signals from a metabolite. In addition, many of lipids in the head region, which may also be obstacles to acquisition of feeble metabolite signals, locally exist immediately under the scalp which is outside the scope of measurement. Therefore, they are suppressed without great difficulty and a preferable spectrum can be obtained easily and stably.
On the other hand, seen from a trend in these few years as to the MRS clinical applications, along with the performance advances of hardware and software of recent years, it becomes possible to stably enhance the static magnetic field homogeneity and to suppress water and lipids with high efficiency. Accordingly, the MRS is increasingly applied to a trunk of the body and four limbs. Recently in particular, breast MRS attracts rising attention, and it is expected that determination of followings will become possible; distinction between benignancy and malignancy of a tumor and prognosis of treatment, by measuring choline (Cho) which reflects exasperation of metabolism in cell membrane.
However, as for the breast MRS, unlike the head MRS, there exist a large amount of lipids inside the measurement region, and therefore, it is critical to suppress lipid signals, so as to obtain a preferable spectrum. By way of example, if only the water signal is suppressed, as performed in a general head MRS measurement but the lipid signals are not suppressed, the Cho signal is superimposed on a tail of enormous lipid signal peak (hereinafter, referred to “a lipid main-band signal”). Therefore, in some cases, a peak shape of the Cho signal itself may be distorted, failing to carry out accurate signal measurement. Furthermore, the lipid signal includes, not only the aforementioned lipid main-band signal, but also a lipid side-band signal, being a peak signal group which is generated in a rippling manner around the lipid main-band signal. The lipid side-band signal is generated due to the effect of the eddy current (GC eddy current), or the like, that the gradient magnetic field generates. When the lipid side-band signals are superimposed on the Cho signal generating band, it is considered as extremely difficult to make a distinction therebetween. For the reasons as described above, it is necessary to suppress the lipid signal with a high degree of precision, in order to perform stable breast MRS measurement.
Until now, there have been suggested various lipid suppression methods, and they fall roughly into four categories as described below:
1) Outside Measurement Region Suppression Method (Pre-Saturation Method)
This method excites in advance, a lipid region in proximity to the measurement region (outside region) before exciting the measurement region, by using a GC pulse for slice selection and an RF pulse for excitation, and thereafter, the method “renders a vector sum of transverse magnetization to zero (spoils the vector sum)” by a GC pulse for phase dispersion.
2) Zero-Cross Method (Zero Cross: a Point of Time when Longitudinal Elements of Inverted Magnetization Vector Sum Temporarily Become Zero)
This method inverts in advance, a lipid signal (proximity band) by using a spectrum selective inversion RF pulse before exciting the measurement region, and applies an RF pulse for exciting measurement region at “the time when the longitudinal elements of the magnetization vector sum during relaxation becomes zero”.
3) T1 Independent Suppression Method (T1: Longitudinal Relaxation Time)
This method applies a spectrum selective inversion RF pulse, before and after applying a region selective inversion RF pulse for inverting transverse magnetization in the region being selected, and “spoils a signal in the selective inversion band (lipid main-band signal)” by phase dispersion/refocusing (positive and negative) GC pulses before and after the spectrum selective inversion RF pulse.
4) TE Shift Averaging Method (TE: Echo Time)
This method repeats the measurement while changing TE, and sums the obtained multiple spectra signals having different TEs, thereby allowing the lipid side-band signals to attenuate.
Among the aforementioned four lipid suppression methods, the method 1) outside measurement region suppression method, is literally a method to perform suppression of incorporated lipid signals entering from the outside of the measurement region, and thus it is not suitable for the case where lipids already exist within the measurement region such as the case of the breast MRS, even though this method is appropriate for the case where the measurement region corresponds to a portion neighboring subcutaneous lipids (brain), such as a head portion. In addition, in the zero-cross method 2), the zero-cross time fluctuates in the case where homogeneous inversion state cannot be obtained, and therefore, it is difficult to perform constantly stable suppression. By way of example, it is not possible to acquire homogeneous inversion state, in the following cases; there are various types of lipids within the measurement region, causing a distribution in T1, and there is inhomogeneity in B1 (a distribution of radio frequency magnetic field) of a transmission radio frequency coil.
The outside measurement region suppression method 1) and the zero-cross method 2) correspond to a “suppression method before main scan” that is previously executed before exciting the measurement region, whereas the T1 independent suppression method 3) is a “suppression method executed during the main scan”. The T1 independent suppression method 3) is a method to selectively inverts and spoils only the transverse magnetization component of the lipid signals, out of the transverse magnetization components generated by the excitation of the measurement region. It is reported that this method is less sensitive to the T1 distribution and B1 inhomogeneities, thereby enabling stable suppression. The T1 independent suppression method 3) includes, for instance, the BASING method for adding a spectrum selective inversion RF pulse having a narrow band characteristic for selectively inverting only a peak of the lipid signals, and phase dispersion/refocusing GC pulses, before and/or after a region selective inversion RF pulse (for example, see the Non Patent Document 1). As for the spectrum selective inversion RF pulse being used here, an RF waveform thereof is optimized by the SLR algorithm, in order to enhance the spectrum selectivity of the RF pulse (e.g., see the Non Patent Document 2).
In addition, the TE shift averaging method 4) is not able to attenuate the lipid main-band signal, but it is able to attenuate a lipid side-band signal which is hard to be attenuated by other lipid signal suppression methods. Therefore, it is reported that the TE shift averaging method 4) is effective when the lipid side-band signal is superimposed on the Cho signal. The TE shift averaging method 4) may include, for instance, TE-Averaging method which repeats measurement while shifting little by little, the time for applying an RF pulse for selecting a voxel to be applied for the third iteration, and the time for starting signal detection, thereby measuring multiple spectra signals with various TEs, and sums the obtained signals as they are (e.g., see the Non Patent Document 3).